A fuel cell system includes a fuel supply system for supplying hydrogen as a fuel to a fuel cell stack and an air supply system for supplying oxygen in the air as an oxidizing agent necessary for an electro-chemical reaction to the fuel cell stack. A heat and water management system of the fuel cell system controls an operating temperature of the fuel cell stack which generates electrical energy by using the hydrogen and air.
The hydrogen is supplied from the fuel supply system to a fuel anode of the fuel cell stack while the oxygen is supplied from the air supply system to an air cathode of the fuel cell stack, so that the hydrogen is oxidized at the fuel anode to generate protons and electrons, in which the hydrogen ions and electrons move to the air cathode through an electrolyte membrane and a separator, respectively. At the air cathode, water is generated through the electro-chemical reaction of the hydrogen ions, the electrons, and the oxygen. The electrical energy is generated through the movement of the electrons.
When a fuel cell vehicle having the fuel cell stack stops, a gas composition at the fuel anode is maintained in the nitrogen atmosphere in which there is almost no oxygen. Then, once the fuel cell vehicle starts again, a constant concentration of hydrogen is required in the fuel cell stack.
When the fuel cell vehicle is prepared to start, a hydrogen purge process for increasing the hydrogen concentration in the fuel stack is performed while exhausting nitrogen from the fuel anode.
The hydrogen purge process reduces a pressure at the fuel anode in the nitrogen atmosphere to smoothly supply new hydrogen to the fuel anode, and simultaneously, supplies the new hydrogen from the fuel supply system to the fuel anode. During the hydrogen purge process, a purge valve connected to an outlet manifold of the fuel anode is opened so that the nitrogen in the fuel anode and a part of the new hydrogen are exhausted to outside through the purge valve.
During hydrogen purge process, it is important to set a suitable purge time. If the purge time is shorter than a reference time, the fuel cell may be shut down due to lack of initial hydrogen concentration. If the purge time is longer than the reference time, although it is possible to secure a sufficient hydrogen concentration in the fuel cell stack, a leakage amount of hydrogen discharged to the outside through the purge valve increases. Thus, the fuel efficiency deteriorates and an additional system is necessary to satisfy the environmental regulations.
Hereinafter, a hydrogen purge process for initial start-up of a fuel cell stack according to the related art will be described in more detail.
FIG. 1 is a schematic view illustrating a hydrogen purge process for a fuel cell stack according to the related art. In FIG. 1, a fuel cell stack 20 includes a plurality of fuel cells stacked therein.
An anode inlet and an anode outlet of the fuel cell stack 20, through which hydrogen is supplied or discharged, are connected to an inlet manifold 10 and an outlet manifold 30.
For reference, manifolds are additionally provided to a cathode inlet and a cathode outlet of the fuel cell stack.
The inlet 12 of the inlet manifold 10 and the outlet 32 of the outlet manifold 30 are opened in the same direction.
An ejector 14 of a fuel supply system is connected to the inlet 12 of the inlet manifold 10, and a purge valve 34 is connected to the outlet 32 of the outlet manifold 30.
The inlet 12 of the inlet manifold 10 and the outlet 32 of the outlet manifold 30 are opened in the same direction. The fuel cell stack 20 includes a first cell 22 which is located proximate to the inlet 12 and the outlet 32 and an n-th cell 24 which is located farthest away from the inlet 12 and the outlet 32.
In the related art, the hydrogen purge process for initiating the fuel cell stack includes supplying hydrogen from the fuel supply system into the inlet manifold 10 through the ejector 14. The fuel anode in the fuel cell stack 20 is filled with the hydrogen from the inlet manifold 10. Residual nitrogen and impurities in the fuel anode are discharged through the purge valve, which is controlled to be opened during the hydrogen purge process, 34.
If the period of the hydrogen purge process, that is, a purge time, is longer than a reference time, although it is possible to secure a sufficient hydrogen concentration in the stack, a leakage amount of hydrogen discharged to outside through the purge valve increases. Thus, the fuel efficiency deteriorates and an additional system is required to satisfy the environmental regulations.
In addition, since a flow rate of hydrogen passing through the first cell 22, which is proximate to the inlet 12 and the outlet 32, in the fuel cell stack 20, is larger than that of the n-th cell 24, a large amount of hydrogen is discharged to outside through the inlet manifold 10, the first cell 22, the outlet manifold 30, and the purge valve 32 in sequence during the hydrogen purge process. Thus, the fuel efficiency deteriorates and an additional system is required to satisfy the environmental regulations.
When the period of a hydrogen purge process is reduced in order to minimize hydrogen discharged to outside due to the above problem, the hydrogen concentration in the stack is reduced, thus causing instability in a fuel cell initial start.
The description provided above as a related art of the present disclosure is just for helping in understanding the background of the present disclosure and should not be construed as being included in the related art known by those skilled in the art.